The increasing incidence of gunfire in urban areas is an unfortunate and troublesome development in many modern cities. In addition to addressing this problem with conventional police measures, strategies for effectively responding to specific gunfire incidents would be useful. In particular, the ability to quickly locate gunshots in an urban area can help quicken the response of law enforcement officials and medical rescue teams to the location of the gunfire. In addition, delayed response by community-oriented police officers can help inhibit repeat offenders.
J. C. Lahr and F. G. Fischer, in the article "Location of Acoustic Sources Using Seismological Techniques and Software," Open-File Report 93-221, United States Geological Survey, Menlo Park Calif., 1993, report of an experiment in the summer of 1992 to locate gunshots using earthquake location software and microphones distributed in an area suffering gunfire. The experiment used as acoustic sensors 5 radio walkie-talkies with an average separation of 700 ft. to transmit signals to a central site where a computer analyzed the signals. Seismological software was adapted to a two-dimensional geometry with a constant sonic velocity. Impulsive events were flagged automatically by the computer and data around those events were saved for analysis later. Typically every 24 hours an operator inspected the data and determined manually the onset times of the 5 sensors for each event. Many potential gunfire locations were found in a 3-month experiment. The experimenters were able to demonstrate on two occasions that sounds from gunfire allowed triangulation to the correct location (from finding a spent shell and a broken window).
Although this proof-of-principle experiment demonstrated the scientific ability to locate urban gunfire, its capabilities did not extend to a reliable and operational system useful for law enforcement dispatch. In particular, the system did not provide an automatic real-time determination of the gunshot locations and times, was not able to reliably discriminate between explosive events and other loud urban noises, did not have an automatic event alerting signal, and did not automatically indicate rapidly the gunfire location on a map to facilitate dispatching.
In U.S. Pat. No. 5,455,868 issued Oct. 3, 1995, Sergent et al. disclose a technique for discriminating gunshot signals from other loud signals. The technique is presumably suitable for short-range detection and alarm, and does not provide a useful system for long-range detection over a large area. Moreover, the rise and fall times used by Sergent to discriminate gunshots are about ten times longer than experimentally determined times of actual gunshots. Consequently, Sergent's method can not properly discriminate gunshots from many other events.
A system for detecting and reporting urban gunshot events is disclosed by Edward Page and Brian Sharkey in "SECURES.TM.: System for Reporting Gunshots in Urban Environments", Public Safety/Law Enforcement Technology, Proc. SPIE 2497, 160-172, 1995. A similar system is disclosed by Sharkey et al. in U.S. Pat. No. 5,504,717 dated Apr. 2, 1996. The system detects potential gunshot signals using a dense grid of "pole units", i.e. a collection of microphones placed on utility poles at every street corner (i.e. about every 500 ft.) in an urban area to be covered. Since this high density grid requires more than 80 sensors per square mile covered, it is very expensive to deploy and maintain. Sharkey does not, however, disclose or suggest any method for reducing the number of pole units required by his system, and overcoming this significant disadvantage.
The acoustic signals at each pole unit are analyzed to identify potential gunshot events. In particular, if a signal at a pole unit is 5 times louder than the average noise level, then an event is reported. A central processor uses the relative time data from reported events to perform triangulation and locate the origin of the gunshot. Sharkey mentions in general terms some signal processing techniques which may be useful in distinguishing a gunshot from other sounds. The mentioned techniques, however, apply only to the analysis of signals from single sensors. That is, these techniques are limited to discriminating gunshots from other types of sounds at each pole unit independently of the other pole units. Notably, Sharkey does not disclose any technique for detecting gunshots through the inter-dependent analysis of multiple signals from multiple pole units. Nor is it at all obvious from Sharkey what specific techniques one might use to perform concurrent analysis of several pole unit signals to improve the reliability of gunshot detection. Sharkey also does not disclose any technique for determining which of several potential triplets of sensors should be used in the triangulation to obtain the best location estimation.